1. Field of the Invention
The present invention relates to fuel combustion in stationary power generating plants; and more particularly to particulate carbonaceous fuel and the use thereof in fluidized-bed combustion processes.
2. Description of the Prior Art
For many years, a goal of designers of combustion equipment has been to provide the capability of extracting a maximum amount of energy from a given fuel. To accommodate this goal, producers of fuel have strived to provide fuel having a relatively high heating value per unit of weight. With respect to particulate carbonaceous fuel, such as anthracite and bituminous coal, this has been generally accomplished by preparing the fuel in a manner as to contain a minimum amount of non-combustible material. In the art, this fuel is referred to as a low ash fuel because, when burned, the amount of ash in the resulting combustion product is a relatively low percentage of the weight of the fuel prior to combustion--generally about 10 percent, but less than about 20 percent, by weight.
Traditionally, low ash coal from a feedstock of mine tailings, or culm, has been prepared by any of several different processing techniques used alone or in combination with one another. For instance, carbonaceous material of relatively large particle size, such as 1 inch by 1/16 inch, may be fed into a cyclonic separator which separates it into a low ash product and a relatively high ash refuse. Such a separator is generally of a so-called tapered configuration and functions in cooperation with magnetite ore to effect separation at a desirable specific gravity and produce the desired product.
In U.S. Pat. No. 4,364,822 entitled "Autogenous Heavy Medium Process and Apparatus for Separating Coal from Refuse", issued to the present applicant, there is disclosed a process for producing a low-ash coal. In the process, the specific, gravity of the separating medium is regulated entirely by the amount of rock or shale contained within the mine tailings in combination with a cyclone of a particular configuration. This eliminates the need for magnetite ore and the cost associated therewith, and results in a product having a relatively low ash content.
Low ash coal is a suitable fuel for a variety of combustion processes, including fluidized-bed combustion processes. Such fluidized bed combustion processes, however, are also capable of burning relatively high ash coal. In a couple of installations of which Applicant is aware, the high ash coal is provided simply by crushing all of the material contained in a culm bank and feeding it directly into the fluidized-bed combustion equipment. The only pre-combustion fuel processing that takes place is that of removing oversize particles that might damage the crushing equipment. Such high ash fuel generally has an ash content of at least about 60 percent by weight. Such high ash coal is not particularly desirable as a fuel because it contains so much non-combustible material that the efficiency of the combustion process is relatively low.
There is often a tradeoff between the cost of producing a given fuel and the efficiency of its utilization. For instance, on the one hand while low ash fuel has a high heating value (BTU/lb.) and can be burned efficiently, it is relatively expensive to produce. On the other hand, while some high ash fuel can be less expensive to produce, its heating value is much lower so that its real cost, i.e. the cost of providing the equipment to burn it and of maintaining the equipment, is higher than desirable. Moreover, the total costs of producing crushed culm fuel can exceed that of producing washed culm fuel on a BTU per pound basis. A desirable fuel, therefore, is one which can be both produced in a cost efficient manner and burned efficiently.
In Applicant's copending U.S. application Ser. No. 860,464 filed May 7, 1986 entitled "Process for Separating High Ash Coal From Refuse", now U.S. Pat. No. 4,795,037, there is disclosed an improved process for producing a high ash carbonaceous fuel particularly suited for use in fluidized-bed combustion equipment. The process utilizes a separating cyclone such as disclosed in U.S. Pat. No. 4,364,822, supra, but in combination with magnetite ore to separate carbonaceous material having a high percentage of ash-forming impurities from non-combustible, ash-forming refuse. Prior to the process disclosed in Applicant's co-pending application Ser. No. 860,464, supra, there was no known cost efficient way of producing a high ash coal product capable of being burned efficiently in a fluidized-bed combustion process.